FIGS. 1a to 1c of the accompanying drawings show a known exhaust gas cooler. This prior art cooler comprises a circular shell 1 fitted with tapered ends 2 which serve as an exhaust gas entry orifice 3 and an exhaust gas exit orifice 4. The orifices 3 & 4 are provided with flange plates 10 for connection to exhaust pipes (not shown). The ends of the shell 1 are sealed by circular tube plates 5 which define a coolant chamber inside the shell 1. Each tube plate 5 has a number of circular holes 6 arranged through it. The holes 6 in each tube plate 5 are connected by a number of small-diameter tubes 7 which are sealed at one end to the first tube plate 5 and at the other end to the second tube plate 5. Exhaust gases flow into the entry orifice 3, along the inside of the small-diameter tubes 7 and out of the exit orifice 4. The exterior of the shell 1 is provided with an entry nozzle 8 and an exit nozzle 9 which respectively supply coolant liquid to and drain coolant liquid from the coolant chamber within the shell 1.
Prior art exhaust gas coolers (such as that shown in FIGS. 1a-1c) are bulky and do not fit easily within the frequently cramped layout of the engine compartment of a road vehicle. The possible positions to fit an exhaust gas cooler within an engine compartment are limited by the fact that the exhaust gases flow into the cooler at one end and flow out at the other end. At certain times, particularly during engine start-up, it is necessary to stop the exhaust gases being cooled. The prior art exhaust gas coolers therefore require special valve and bypass tube arrangements so that exhaust gases can be diverted around the exhaust gas cooler when cooling is not required.